Laser doping of silicon in localised regions beneath metal contacts, has been proposed for more than a decade as a low cost approach for potentially producing high performance solar cells with selective emitters. To date, despite more than a decade of research and problem solving, devices using laser doping in conjunction with an anti-reflection coating, have not achieved their expected performance due to defects, junction recombination or shunting arising from the laser doping process. In particular, defects adjacent to the melted regions resulting from the thermal expansion mismatch between the silicon and the overlying antireflection coating (ARC), inadequate mixing of the dopants incorporated into the molten silicon and unwanted ablation of the doped silicon are significant problems contributing to the poor electrical performance of devices using laser doping of localised areas beneath the metal contacts.
Further, most solar cells use an anti-reflection coating (ARC) on the semiconductor surface to reduce the amount of light reflected. The ARC is usually chosen to have the right refractive index and thickness so as to reduce the surface reflection to a minimum. A double layer ARC (DLARC) could also be used whereby the refractive index and thickness of each individual layer is chosen to reduce the overall reflection to a minimum, with the theoretical reflection minimum for a DLARC being below the theoretical minimum for a single layer ARC (SLARC). Most commercially manufactured solar cells use a SLARC as it is too complex and expensive to use a DLARC for the small additional benefits in performance.
Two problems that can result from using an ARC are: firstly, an ARC may make it difficult to passivate the semiconductor surface onto which it is deposited, therefore leading to increased recombination and device dark saturation current; and secondly, many potential ARC materials will have different thermal expansion coefficients to the semiconductor material onto which it is deposited, leading to stressing of the semiconductor surface with possible corresponding defect generation during treatment at elevated temperature. To overcome the first, surface treatments such as the growth of a thin thermally grown oxide layer to passivate the semiconductor surface prior to the deposition of the much thicker ARC have been used. With this arrangement the thin passivation layer does not significantly affect the operation of the ARL deposited over it.
However to date, a suitable solution does not appear to have been proposed for simultaneously achieving good ARC properties while simultaneously providing thermal expansion mismatch correction for the ARC and passivating the semiconductor material and surface. In reality, a high performance solar cell technology to be viable commercially needs to be able to use an ARC that performs all three functions while simultaneously being able to be deposited in a simple low cost process.